The use of short, user-selected (i.e., not defined by the target nucleic acid(s)) nucleic acid sequences (also known as “tags”) is a very powerful technique for the design and development of novel nucleic acid assay formats. These nucleic acid formats include nucleic acid amplification, sequencing or other assay formats.
Nucleic acid amplification provides a means for making more copies of a nucleic acid sequence that is relatively rare or unknown, for identifying the source of nucleic acids, or for making sufficient nucleic acid to provide a readily detectable amount. Amplification is useful in many applications, for example, in research, diagnostics, drug development, forensic investigations, environmental analysis, and food testing.
Typically, nucleic acid amplification uses one or more nucleic acid polymerase and/or transcriptase enzymes to produce at least one copy, and preferably multiple copies of a target nucleic acid sequence and, optionally, a tag sequence.
Many methods for amplifying nucleic acid sequences in vitro are known, including polymerase chain reaction (PCR), ligase chain reaction (LCR), replicase-mediated amplification, strand-displacement amplification (SDA), “rolling circle” types of amplification, and various Transcription Mediated Amplification (TMA) and reverse TMA (rTMA) methods. These known methods use different techniques to make amplified sequences, which usually are detected by using a variety of methods. See, for example, Schweitzer and Kingsmore, combining nucleic acid amplification and detection, current opinion in Biotechnology, 2001, 12, 21-27. These methods can be exemplified by the following publications (each of which is hereby expressly incorporated by reference): PCR—U.S. Pat. Nos. 4,683,195, 4,683,202, and 4,800,159; LCR—U.S. Pat. No. 5,516,663 and EP 0320308 B1; Replicase-mediated amplification—U.S. Pat. No. 4,786,600; SDA—U.S. Pat. Nos. 5,422,252A and 5,547,861; Rolling circle types of amplification—U.S. Pat. Nos. 5,714,320 and 5,834,252; TMA—U.S. Pat. Nos. 4,868,105, 5,124,246, 5,130,238, 5,399,491, 5,554,516 and 5,437,990, 5,824,518, US 2006-0046265 A1 and WO 1988010315 A1; rTMA—US 2006-0046265.
Amplification methods may introduce nucleic acid sequences into the sequence being amplified. Some methods use modified primers to introduce non-target nucleic acid sequences to the sequence being amplified. One example of a modified primer is a “tag” primer. A tag primer contains two parts: (1) a “tag sequence” that is a nucleic acid sequence that does not hybridize to the target nucleic acid sequence and (2) a primer sequence that is a nucleic acid sequence that does hybridize to the target nucleic acid sequence. The tag sequence is located 5′ to the primer sequence. The first round of amplification incorporates the tag sequence into the sequence being amplified. The second round of amplification uses primers that are complementary to the tag sequence.
Anchored PCR is a modified PCR method that uses an “adaptor” primer to amplify a sequence which is only partially known. See, for example, Loh et al., 1989, Science 243(4888):217-20; Lin et al., 1990, Mol. Cell. Biol. 10(4):1818-21).
Nested PCR may use primer(s) that contain a tag sequence unrelated to the target nucleic acid's target sequence to amplify nucleic acid from unknown target sequences in a reaction. See, for example, Sullivan et al, 1991, Electrophoresis 12(1):17-21; Sugimoto et al., 1991, Agric. Biol. Chem. 55(11):2687-92.
Other forms of amplification use a probe or probe set to introduce non-target priming sites located upstream and downstream of a target-specific sequence and adapter sequence(s). See, for example, U.S. Pat. Nos. 6,812,005 and 6,890,741, Fan et al. The two probes that bind in close proximity on a target sequence may be ligated together before being amplified by using the upstream and downstream universal priming sites.
Alternative assay methods may use probe hybridization and linear signal amplification by using a common sequence that is included in a variety of target nucleic acid-specific probes (e.g., US 20070111200, Hudson et al.). This method uses a labeled cassette that contains a sequence complementary to the common sequence to detect multiple target nucleic acids.
One problem that has heretofore existed with the use of tags is the lack of a rigorous selection method to identify the best tag for a given application. This has resulted in the use of less than optimal tags for particular applications.
Another problem is that tags may engage in undesired cross-reactions with other tags, primers, promoter providers, probes, amplicons, other target and non-target sequences and other such sequences in a given assay.
Another problem is that tags may engage in undesired cross-reaction with sequences in test samples from known or unknown sources, such as pathogenic or non-pathogenic organisms, mammalian nucleic acids, contaminating nucleic acids from enzymes, side-products of nucleic acid amplification reactions, etc.
One of the unsolved problems with multiplexed detection or amplification is the interference observed during multiplexing experiments, which limits the dynamic range, precision, and quantification characteristics for the target nucleic acids when present together in a sample.
In multiplex amplification reactions, a variety of undesired “side reactions” can occur that ultimately degrade assay performance. For instance, in the Transcription-Mediated Amplification (TMA) context, primers and/or promoter based amplification oligomers directed towards one target nucleic acid (or group of target nucleic acids) can interact with primers and/or promoter based amplification oligomers directed towards another target nucleic acid (or group of target nucleic acids), causing degraded performance of one, or the other or both amplification systems. This problem typically gets worse as the number of amplification systems present in a multiplex reaction increases. Other interactions that reduce assay performance include amplification oligomers interacting with one another or with other oligonucleotides in the amplification reaction, such as probes, blockers and target capture oligomers (TCOs), and amplicons. This problem of negative interaction between nucleic acids in a system can be reduced or solved by either converting all of the target specific primers/promoter providers in the assay or a portion of the target specific primers and/or promoter providers in the assay into primers and/or promoter based amplification oligomers containing tag sequences. Early rounds of amplification take place using these tagged amplification oligomers, thereby incorporating the tag sequences and their complements into the early amplification products. Subsequent rounds of amplification can take place using primers and/or promoter-based amplification oligomers having target hybridizing sequences directed to the incorporated tag sequence. In this way, the make-up of the subsequent round amplification oligomers is controlled by the user and undesired side reactions are reduced or eliminated. It is notable that the tag sequences used in two or more separate amplification oligomers can be the same sequence or different sequences.
Another related problem is the lack of a convenient procedure to quantitatively or qualitatively measure the relative levels of interferences in a multiplexed reaction.
Additionally, competition for amplification reaction resources may occur in multiplex amplification reactions when the same tag sequence is used for multiple tagged amplification oligomers. For example, in multiplex amplification reactions, target nucleic acid present at high levels will consume the amplification oligomers complementary to the tag sequence much faster than target nucleic acid present at low levels. The inability to uniformly amplify the target nucleic acids due to amplification resource competition may lead to false negatives because the target nucleic acids present at low levels are not amplified to a detectable amount.
One possible solution to these problems is to create unique tag sequences for incorporation into one or more oligomers in an assay. The unique tags are designed such that they do not interact with each other, or with any other sequences in the assay. In this way, an assay incorporating one or more tag sequences can proceed without reduced performance caused by the undesired interaction of various nucleic acids, including tag sequences, present in the assay reaction.
Clearly, there are numerous problems in the art of using nucleic acid tag sequences in a nucleic acid assay. It would be desirable to have tag sequences and methods for identifying tag sequences that are useful in a nucleic acid assay and that avoid the problems. It would be desirable to have methods for rapidly identifying tag sequences for use in a nucleic acid assay.